Power demand from an electric system can vary considerably. In order to improve the efficiency of an electric system, it is desirable to store excess, off-peak, and renewably-generated electricity so that the stored electricity can be utilized when demand is high. There are several available methods for storing energy which is later used to produce electricity, including batteries, elevated hydro systems, and compressed air energy storage (CAES) systems.
CAES systems compress atmospheric air in a compressor driven by energy from the electric system. The compressed air is stored in a compressed air reservoir, e.g., a geological formation or other structure. When the energy is demanded, the compressed air can be heated and expanded to generate electricity. Various devices can be used to compress and expand the air for the CAES system. For example, a positive displacement machine (PDM), such as a typical internal combustion engine, reciprocating air compressor, or rotary displacement device, can compress air for storage. One of the cost reduction methods for CAES systems is to use a PDM in a bidirectional manner for both the compression and expansion processes. However, bidirectional PDMs are often mechanically complicated and tend to operate at high pressure ratios, causing high temperature changes in the system. This can result in a relatively low amount of recovered energy. As a result, there exists a need for an efficient, low-cost, bidirectional (e.g., reversible) compressor/expander for use in a CAES system.